For anyone having similar problems, I've discovered that when choosing Spotify as the music widget in minimalist mode, there are two problems:
1: Often the widget will appear to have loaded but will not work - I'll need to open Spotify in the background and then it works as expected.
2: When the widget is working, it is difficult to move within the track by touching the bar - I'd have to click it multiple times to get it to register.
Both problems were solved by setting Spotify to be one of the three default apps instead of the music widget, then loading into minimalist mode that way. This allows for seeing a lot more data (switching between views), and it's much easier to move forward or backward within a track. The downside with this method is that occasioanlly Spotify will load with the menu on the left (like a tablet view instead of a phone view) and I'll need to expand the window size and reduce it again, which fixes the display issue. That said, it's better than using the widget.
I still can't use the steering wheel controls to advance the track, but that's less important now that the app is working as expected.